1. Field of the Invention
The present invention generally relates to computer chip manufacture and, more particularly, to improving the manufacture of interconnect structures in the form of solder bumps used in "flip chip" interconnects, by removing oxide that occurs during the manufacturing process.
2. Background Description
"Flip-chip" interconnects comprise an array of solder balls deposited on metallic layers on one side of an integrated circuit (IC) chip. These structures are sometimes referred to as Controlled, Collapse, Chip Connection (C4) structures, and the solder balls are designed to mate with metallic pads on a chip carrier or substrate having metallized wiring. In the practice of the present invention, the metallic layers, called Ball Limiting Metallurgy (BLM), on which the solder balls are formed comprise layers of titanium-tungsten and phased chromium-copper/copper.
Titanium-tungsten is used as an adhesion layer which electrically contacts interconnects formed in a surface of the chip. The process steps in electrochemical fabrication of complete "flip-chip" interconnects involve:
1. Sputter deposition of seed layers (titanium-tungsten and phased chromium-copper/copper); PA1 2. Application of photoresist and its patterning; PA1 3. Electrodeposition of solder; PA1 4. Stripping the photoresist; PA1 5. Etching of the seed layers; and PA1 6. Reflow.
The seed layer etching is a two step process that consists of first removing the copper and chromium-copper by electrochemical etching and then removing the titanium-tungsten by chemical etching. The seed layers which remain underneath the solder balls after the two step etching process forms the BLM.
The chemical etching of the titanium-tungsten is preferably performed by a hydrogen peroxide (H.sub.2 O.sub.2) based etchant. The chemical etchant has to first remove an oxide film which forms on the titanium-tungsten during the electrochemical etching of the copper and chromium-copper before etching the actual titanium-tungsten metal layer. The variation in the oxide film across the wafer as well as from wafer to wafer leads to differences in the time required for removing the oxide film and initiation of the actual titanium-tungsten metal removal. Since the oxide etch rate differs from the metal etch rate, the result is non-uniform etching and wide variations in the amounts of titanium-tungsten undercuts.
The resulting cross-sectional profile of the BLM structure is critical to building a sound and robust interconnect pad. The undercut or process bias of the titanium-tungsten layer is determined during the wet chemical etch process. The resulting diameters of the copper and phased chromium-copper layer are easily measured under a microscope on test sites or partial chips. The resulting diameter of the titanium-tungsten portion of the BLM structure is not so easily measured because it is hidden by the overlaying phased chromium-copper layer. Excess titanium-tungsten undercut must be controlled by process design in order to ensure a mechanically sound BLM structure.
The titanium-tungsten seed layer of the BLM is typically on the order of 1000.ANG. and consists of 90% tungsten and 10% titanium. Titanium-tungsten undercuts due to wet etching have been measured to be in a range from 1 micron to 9 microns relative to the phased chromium-copper layer of the BLM. The variation in the titanium-tungsten undercut is seen across the wafers as well as from wafer to wafer. Factors affecting the amount of titanium-tungsten undercut include etch solution composition, titanium-tungsten layer thickness, and amounts of etch resistant metal oxides formed on the titanium-tungsten surface.
The amount of titanium-tungsten undercut cannot be measured nondestructively. Measuring of titanium-tungsten undercut can also significantly add to the cost. Therefore, a seed layer etching process that ensures minimal titanium-tungsten undercuts (e.g., less than or equal to 1 micron) is highly desirable.